I would ask your local building codes department about the type of CSST you have or want to use.
And you may need license or certification to do it yourself.
A short blurb about this...
Installation of CSST is not allowed underground (directly buried) unless it is of a type specially designed and listed for underground applications [see Commentary
Figure G2414.5.3(9)]. Some manufacturers’ installation instructions allow CSST to be “indirectly” buried by installing the tubing within a nonmetallic, water-tight conduit, such as PVC, PE or ABS plastic pipe. Other manufacturers may offer CSST with an integral casing and sealing fittings specifically designed for underground installation. See
Sections G2415.6 and
G2415.12 through
G2415.14 regarding piping buried in slabs and beneath buildings.